A synthetic zeolite is crystalline aluminosilicate, and has uniform pores with angstrom size that are attributed to a crystal structure. Taking advantage of such a feature, the synthetic zeolite is industrially used as a molecular sieving adsorbent that adsorbs only a molecule having a specific size, an adsorption separating agent that adsorbs a molecule having strong affinity, and a catalyst material. A beta zeolite that is one of such zeolites is currently used in quantity as a catalyst in the petrochemical industry or an adsorbent for vehicle exhaust gas treatment throughout the world. The characteristic of the beta zeolite is that the zeolite has pores of a 12-membered ring in a three-dimensional direction, as disclosed in Non Patent Document 1. Moreover, the X-ray diffraction pattern showing its structural characteristics is disclosed in Non Patent Document 2.
A method of synthesizing the beta zeolite is variously proposed. A typical method is a method of using a tetraethylammonium ion, as an organic structure-directing agent (hereinafter abbreviated to “OSDA”). Such methods are disclosed in, for instance, Patent Documents 1 to 3 and Non Patent Document 3 below. According to these methods, the beta zeolite having a SiO2/Al2O3 ratio of 1 to 400 can be obtained. However, a compound containing the tetraethylammonium ion is expensive, and what is more, most of the compound is decomposed after beta zeolite crystallization is terminated. As such, it is impossible to recover and reuse the compound. For this reason, the beta zeolite produced by such a method is expensive. Furthermore, since the tetraethylammonium ion is incorporated into a crystal of the beta zeolite, it is necessary to calcine the beta zeolite to remove the tetraethylammonium ion when the beta zeolite is used as the adsorbent or the catalyst. In that case, exhaust gases are responsible for environmental pollution, and further, many chemicals are required for detoxification treatment of a synthetic mother liquid. In this way, the method of synthesizing the beta zeolite using the tetraethylammonium ion is not only an expensive method but also a production method having a great environmental load. As such, there is a need to realize a production method of using no OSDA.
Under such circumstances, a method for producing a beta zeolite using no OSDA has been proposed in Non Patent Document 4. In this method, the beta zeolite synthesized using tetraethylammonium ions is calcined to remove organic components and the obtained product is used as a seed crystal. The seed crystal is added to a sodium aluminosilicate reaction mixture containing no organic substance, a hydrothermal treatment is performed, and thereby crystallization is performed. However, in this method, as long as the beta zeolite synthesized by using tetraethylammonium ions is calcined and the obtained product is used as a seed crystal, the tetraethylammonium ions as OSDA are always needed, though the used amount is reduced. Moreover, in the above-mentioned document, only one type of seed crystal is disclosed, and regarding the composition of the sodium aluminosilicate reaction mixture, only one example is shown in which numerical values are limited. Therefore, although the composition of the synthesized beta zeolite is not disclosed in the above-mentioned document, it is considered that the composition has only determined values.
On the other hand, Patent Document 4 by the authors of Non-Patent Document 4 discloses a SiO2/Al2O3 ratio of a seed crystal, and further Patent Document 4 discloses a composition of a sodium aluminosilicate reaction mixture not as a single discrete composition but as a narrow range which differs to defining the mixture as a single discrete composition. However, the technique disclosed in Patent Document 4 is basically the same technique as that of Non-Patent Document 4, and the range of the composition of the reaction mixture is so narrow that the SiO2/Al2O3 ratio of the zeolite is limited to only a limited range. In order to meet various demands, a zeolite having a wide range of SiO2/Al2O3 ratio is desirable. Moreover, establishment of a condition allowing synthesis by stirring is desired for industrial mass production. In order to reduce the environmental load as much as possible, a proposal of a new process for producing a beta zeolite is desired, in which a seed crystal which need not be calcined is used and no OSDA is used.
Thereafter, as described in Patent Document 5 and Non-Patent Document 5, conditions under which the synthesis can be performed in a wider composition range of the reaction mixture were found by the present inventors, and the SiO2/Al2O3 ratio of the obtained beta zeolite was also expanded. In addition, Patent Document 5 and Non-patent Document 5 disclose a method for synthesizing a beta zeolite using no OSDA, wherein a beta zeolite is synthesized by adding a seed crystal without using an OSDA and further the synthesized beta zeolite is added again as the seed crystal by way of recycling. Since this method is a green process in which no OSDA is essentially used and environmental load is ultimately small, it is possible to synthesize a so-called “green beta zeolite” by this method.
Patent Document 1: U.S. Pat. No. 3,308,069
Patent Document 2: U.S. Pat. No. 4,923,690
Patent Document 3: Japanese Unexamined Patent Application, Publication No. H09-175818
Patent Document 4: Chinese Published Patent Application Publication, No. 01249968, Specification
Patent Document 5: PCT International Publication No. WO2011/013560
Non Patent Document 1: Ch. Baerlocher, L. B. McCusker, D. H. Olson, Atlas of Zeolite Framework Types, Published on behalf of the Commission of the International Zeolite Association, 2007, pp. 72-73
Non Patent Document 2: M. M. J. Treacy and J. B. Higgins, Collection of Simulated XRD Powder Patterns for Zeolites, Published on behalf of the Commission of the International Zeolite Association, 2007, pp. 82-83 and p. 480
Non Patent Document 3: Microporous Materials, Vol. 5, p. 289-297 (1996)
Non Patent Document 4: Chemistry of Materials, Vol. 20, No. 14, pp. 4533-4535 (2008)
Non Patent Document 5: Chemistry—An Asian Journal, Vol. 5, pp. 2182-2191 (2010)